The invention relates to a differential transmission comprising a differential casing driven by an input shaft and having compensating gears mounted therein, and in each case having an output gear to drive a first and a second output shaft, it being possible for the differential casing to be connected to one of the output shafts via a friction clutch by pressure being applied to a pressure plate.
By means of the friction clutch, the differential transmission can be locked. This locking action can be both xe2x80x9con/offxe2x80x9d and also continuously adjustable. For the purpose of locking, the coupling is actuated, which requires considerable actuating forces. These forces are normally applied hydraulically. However, hydraulic actuation is too slow to disengage the clutch in the short switching time necessary for electronic slip limitation. In addition, passenger automobiles do not have a hydraulic system. Non-hydraulic actuation, by means of electric means and mechanical transmission, has hitherto failed on the high actuating forces which had to be introduced into the casing via bearings.
In addition, in the case of electrical actuation, the question is always posed regarding operational safety in the event of failure of the electric means. If a lockable differential is used in a transfer box of an all-wheel drive vehicle with an off-highway gear stage, there is the additional problem of controlling this as well so that it is still coordinated with the differential lock.
It is therefore an object of the invention to provide a lockable differential which avoids the disadvantages listed above; no actuating forces are to be introduced into the casing, the short switching times required for electronic slip limitation should be achievable, and all that with the smallest possible outlay on construction and in an inherently safe manner.
According to the invention, the foregoing object is achieved wherein
a) for the axial displacement of the pressure plate, two rings decoupled in terms of rotation with balls between them are provided, of which at least one has ramps that rise in the circumferential direction,
b) both rings each have a ramp lever whose free end interacts with a control unit, which has electric drive means and moves the ramp levers in opposite directions.
The ring rotationally decoupled from each other and from the shaft bearing them can be rotated with respect to each other by means of their ramp levers, no actuating forces having to be dissipated to the casing, because of their opposite movement. The rising ramps in the ring or rings require only a very small relative rotation for high actuating forces, so that, in interaction with the electrical actuation, very short switching times are achieved.
In preferred embodiments, the control unit has a control disk that be rotated by means of a geared electric motor, and the axis of rotation of the control disk is aligned in the direction of movement of the free ends of the two ramp levers.
The invention also relates to a transfer box for motor vehicles, having a differential transmission as claimed in one of the preceding claims and having an off-highway gear stage which can be engaged as a result of axial displacement of one of its elements. In order also to control the off-highway gear stage in as coordinated a manner as possible, with the lowest effort and intrinsically safely, the invention provides that
a) the control unit comprises a carrier shaft driven by the geared motor, a changeover sleeve which is firmly connected to said carrier shaft so as to rotate with it but is displaceable, the control disk rotatably mounted on the carrier shaft and a control pinion, likewise rotatably mounted on said carrier shaft, for changing over the off-highway gear stage,
b) it being possible for the changeover sleeve to be displaced by means of an electromagnet and the changeover sleeve having first and second coupling teeth, by means of which it produces the drive connection to the control disk or to the control pinion as desired.
With this configuration, using a single control unit and a single motor, both the differential lock and the off-highway gear stage can be driven as desired in an intrinsically safe manner. In addition, costs are saved, and also overall space, which is tight in a transfer lock. Furthermore, it is ensured that the two cannot be selected at the same time and that, in the event of failure of the control unit, the lock is released.
In an advantageous development, the electromagnet is a three-position magnet, which displaces the changeover sleeve by means of a selector fork, and the changeover sleeve has switching teeth with a repellent engagement angle. As a result, in the event of a failure of the electromagnet, the coupling disengages automatically, with the effect of intrinsically safe actuation.
Furthermore, in that differential casing, in addition to the compensating gears, planet gears of the off-highway gear stage are mounted, which mesh firstly with a drive sunwheel and secondly with an internal gear, it being possible for the internal gear to be displaced in the axial direction.
In an advantageous development, in each case a locking disk with at least one locking recess is firmly connected to the hub of the control disk and with the control pinion, and interacts with a locking element that is coupled to the changeover sleeve, which locking element has a locking tooth on each side, so that the locking tooth engages in the locking recess in the locking disk when the control disk is disengaged, and so that the locking tooth engages in the locking recess in the locking disk when the control pinion is disengaged. In this way, without any external action, the inherent safety is produced in that the respectively disengaged output element is locked and can be disengaged only after a shifting action has been completed.